Commercial gallium nitride (GaN) is typically synthesized through the reaction of gallium oxide (Ga2O3) with ammonia (NH3), which results in GaN with a high concentration of residual oxygen. High impurity concentrations increases the density of dislocations in GaN single crystals and inhibits continuous growth. Commercial powders made from metal gallium (Ga) are extremely expensive and the quantity available is limited because the yield of current processes is low.
Gallium nitride in the wurtzite form has a large direct band gap of 3.45 eV at room temperature. GaN also forms a continuous range of solid solutions with aluminum nitride (AlN) (6.28 eV) and indium nitride (InN) (1.95 eV). Thus, Group III-nitrides, and gallium nitride in particular, are very attractive for use in opto-electronic devices because any band gap with energy in this range, from the visible to the deep UV, is theoretically achievable with these materials. Opto-electronic devices are currently fabricated by growing GaN on hetero-substrates such as sapphire, SiC, ZnO, Al2O3, LiGaO2, and LiAlO2. The mismatches in lattice parameters and thermal expansion coefficients result in high dislocation densities in these films.
Group III nitrides are widely known to emit light ranging from UV to the visible spectra. Many group III nitrides therefore show potential as opto-electronic light sources (S. Nakamura, et al., Jpn. J. Appl. Phys. 1996, 35, L217) and high frequency power devices (M. A. Kahn, et al. Appl. Phys. Lett. 1993, 63, 1214). For example, gallium nitride powder can be used to make optical displays by utilizing the fluorescent properties of GaN crystals (see K. Hara, U.S. Patent Application Publication No. 2003/0226497 A1). Group III nitrides can be valuable in combination with ceramics, particularly the aluminum nitrides (see “A study of the physical properties and electrochemical behavior of AlN films”, F. Vacandio, Y. Massiani, P. Gravier, Surf Coat. Tech. 1997, 92, 221).
In order to continue and speed the development of this technology, bulk substrates are greatly desired. Bulk substrates will allow the realization of high device efficiencies, lower leakage currents, and longer device lifetimes. GaN bulk single crystals and wafers are thus greatly desired for homoepitaxial film growth. Crystal growth processes for GaN bulk growth, e.g., sublimation and high-pressure methods, demand the availability of a high purity, single-phase GaN powder.
Johnson et al. first presented the synthesis of GaN powder by flowing NH3 over molten gallium at temperatures of 900-1000° C. (W. C. Johnson, J. B. Parsons, M. C. Crew, J. Phys. Chem. 1932, 36, 2651). Later, Lorenz developed a synthetic method of preparing GaN using Ga2O3 and NH3 (M. R. Lorenz, B. B. Binkowski, J. Electrochem. Soc. 1962, 109, 24). In 1972, Pichugin and Yas'kov (I. G. Pichugin, D. A. Yaskov, Inorg. Mat. 1972, 6, 1732) showed that GaN powder could be synthesized via the reaction: Ga(l)+NH3(g)=GaN(s)+3/2H2(g) where ΔGr=−51 kJ/mol at 800° C. Compared to the conversion from Ga2O3, the key advantage of this procedure is that GaN powder of higher purity can be obtained. GaN powder has also been synthesized by injecting NH3 into molten Ga under atmospheric pressure in the temperature range of 900-980° C. (M. Shibata, T. Furuya, H. Sakaguchi, S. Kuma, J. Crystal Growth 1999, 196, 47). There is only one report of the efficient conversion of Ga to GaN with flowing NH3 in a horizontal furnace (C. H. Balkas, R. F. Davis, J. Am. Ceram. Soc. 1996, 79, 2309).
Several research groups have demonstrated the growth of GaN powders through various other synthetic methods. Ga2O (C. M. Balkas, R. F. Davis, J. Am. Ceramic Soc. 1996, 79, 2309), and GaP or GaAs (A. Addamino, J. Electrochem. Soc. 1961, 108, 1072) have been used as precursors and reacted with NH3 to make GaN powder. Other methods have also been explored recently, such as the decomposition of organometallic compounds containing Ga and N (K. H. Kim, C. H. Ho, H. Doerr, C. Deshpandey, R. F. Bunshah, J Mater. Sci. 1992, 27, 2580), plasma synthesis (K. Baba, N. Shokata, M. Yonezawa, Appl. Phys. Lett. 1989, 54, 2309), and microwave-assisted combustion methods (B. Vaidhyanathan, D. K. Agrawal, R. Roy, J. Mater. Res. 2000, 15, 974). Junko disclosed a method of obtaining GaN powders by dropping an ammonia solution into gallium nitric hydrate solution to form gallium hydroxide, followed by dehydration to form gallium oxide, which was then heated under an ammonia atmosphere (Junko et al., Japanese Patent Application No. 10-373540). These processes, however, do not disclose the advantages of using a wetting agent, and more efficient and suitable processes are required by industry. Additionally, most of these methods experience either very low yields or high impurity concentrations.
Cahn first introduced the idea of interfacial wetting in 1977 (J. W. Cahn, J. Chem. Phys. 1977, 66, 3677). Cahn suggested that one component in a binary liquid mixture might segregate to the liquid surface. Recently, interfacial wetting has been observed in fluid alkali metal-alkali halide and liquid gallium-based binary alloys, such as Ga—Bi (E. B. Flom, M. Li, A. Acero, N. Maskil, S. A. Rice, Science 1993, 260, 332; H. Tostmann, E. DiMasi, O. G. Shpyrko, P. S. Pershan, B. M. Ocko, M. Deutsch, Phys. Rev. Lett. 2000, 84, 4385) and Ga—Pb (D. Chatain, P. Wynblatt, Surf. Sci. 1996, 345, 85). Bismuth has been used as surfactant in growing GaN thin films by a Molecular Beam Epitaxy (MBE) technique disclosed by Kisielowski in U.S. Pat. No. 6,379,472. Nickel-mesh has been used as a catalyst to help decrease the kinetic barrier to forming GaN (K. S. Nahm, S. H. Ahn, S. H. Lee, IPAP Conf. Series 1, 2000, 30). None of these studies, however, disclose the use of bismuth, germanium, tin, or lead as a wetting agent for the preparation of Group III nitrides.
Currently, there is a need for bulk supplies of Group III nitrides, particularly high purity Group III nitrides. Few companies in the US manufacture commercial gallium nitride (GaN) powder, and the commercial powders typically contain relatively high levels of impurities. Because of the lack of highly efficient methods for the synthesis of Group III nitrides, novel preparations are highly desirable.
Additionally, recent demonstrations of visible (blue, green, red) and infrared (1.54 μm) electroluminescence from rare earth (RE) doped gallium nitride produced significant interest in Group III nitrides for potential applications in optical communications and full color displays (A. J. Steckl and J. M. Zavada, Mater. Res. Bull. 1999, 24, 33; A. J. Steckl, et al. Compound Semicond. 2000, 6, 48). The properties of erbium-doped GaN and other III-V semiconductors have been reviewed by Zavada and Steckl (J. M. Zavada and D. Zhang, Solid-State Electron. 1995, 38, 1285; A. J. Steck, et al. IEEE J. Selected Topics in Quantum Electronics, 2002, 8, 749) The GaN-based semiconductor structures are of great interest because they appear to be optically robust, and chemically and thermally stable. Favennec et al. reported a strong dependence of the emission intensity of the Er3+ ions on the band gap of the host semiconductor and on the material temperature (P. N. Favennec, et al. Electron. Lett. 1989, 25, 718). The wide band-gap of GaN leads to a reduced RE emission quenching effect and the observation of strong RE emission at room temperature (ibid). Accordingly, novel preparations of rare earth metal-doped Group III nitride compositions are needed.